Not Applicable.
1. Field of the Invention
The invention relates to a hybrid system built up with hydraulic and electric elements for motion control of dynamic machines. A cylinder linked to a pump by means of an open hydraulic circuit is used for power transmission while power control is dedicated to a variable speed electric motor.
2. Discussion of Related Art
The lasting use of resources has become a key issue in modern industrial development. There is increasing awareness, that air, soil, water and energy are limited resources. Strict laws regarding the use of energy and emissions of all kinds are generally implemented, and are already a decisive factor in the assessment of machines. This trend is going to become more pronounced in the future (Energy taxes, CO, emission taxes, ISO 14000, EC Machine Guidelines, etc.). With this in mind, it is important for hydraulic drive technology to improve energy efficiency and to offer solutions comparable to those of the competing electrical systems. Otherwise, certain applications constituting the strength of hydraulics could disappear.
A typical example are plastic injection molding machines, traditionally the domain of hydraulic drive technology. xe2x80x9cAll electricxe2x80x9d machines are making an appearance on the market. Their main claim is energy consumption and control performance of machine movements. Since xe2x80x9call hydraulicxe2x80x9d machines are also making progress in energy consumption, they have not yet been displaced from the market. The parallel existence of both drive types shows that there are arguments in favor of both sides.
The goal of the invention was to find a hybrid solution using the advantages of both drive technologies rather than following an xe2x80x9call hydraulicxe2x80x9d or xe2x80x9call electricxe2x80x9d approach. The solution found consists of a variable speed electric motor, a pump, an open hydraulic circuit and a hydraulic actuator such as a cylinder or a hydromotor. The invention shows only little differences when compared to similar concepts, but offers new features, which make it more efficient, more controllable and more versatile.
Hydraulic and electric actuators show very different properties. An advantage of hydraulic drive technology is the possibility for one motor-pump setup to share power with a multitude of actuators. In open hydraulic circuits, one pump supplies pressurized oil or other fluids to a multitude of actuators (please find a description of an open hydraulic circuit later in the text). Each actuator is assigned a valve controlling the motion of the respective axis. For certain machines only one electric motor is required even though the machine has multiple axes installed. This results in saving considerable costs compared to all electric machines that require one motor for every axis of the machine. For electric machines, power sharing is not as easy to achieve and therefore most often not applied.
High force concentration is another advantage of hydraulic technology. Electric motors and also linear motors generate much lower torque or forces than their hydraulic equivalent. Hydraulic motors and cylinders show about 10 times higher force to surface ratio (force concentration). This is due to the fact that different physical principles are behind electric and hydraulic forces. To achieve higher forces xe2x80x9call electricxe2x80x9d machines often require additional transmission elements such as ball screws, gear boxes, belt drives or toggle mechanisms where as hydraulic actuators can act directly on the machine. This issue becomes an important factor for machines generating high forces such as presses or injection molding machines.
An advantage of electric drive technology is power control. Power transistors such as IGBT (insulated gate bipolar transistor) can switch on and off in as little as 100 nanoseconds (10{circumflex over ( )}xe2x88x929 sec). This is about 100,000 times faster than hydraulic valves where typical switching times are 10 ms (10{circumflex over ( )}xe2x88x923 sec). Fast transistor technology allows controlling electric energy with little losses. It also allows recuperating mechanic energy back into electrical energy. The associated drive technologies are state of the art and well known.
Hydraulic power control, on the other hand, is based on valves or variable displacement pumps. Hydraulic control valves are variable resistors changing the pressure drop in direction of the flow. This results in high losses of energy. Note that within the scope of this application we distinguish between control valves and on/off valves. Control valves can be understood as variable orifice allowing to control the flow supplied to an hydraulic actuator. The orifice causes resistance to the oil flow resulting in a pressure drop. The product of pressure drop and flow represents power losses inherent to this way of controlling power. On/off valves on the other hand are used to direct the flow from one line to the other or to switch the flow on and off. They can not be used to control the motion of a hydraulic cylinder or hydromotor. When xe2x80x9coffxe2x80x9d, there is no flow through the valve, when xe2x80x9conxe2x80x9d there is virtually no pressure drop in the valve as these valves do not hinder the oil to flow (no orifice effect). Accordingly, power losses are virtually zero, flow and pressure drop always multiply to zero. For this invention, I therefore tried to avoid the use of control valves while on/off valves do not waste energy and therefore can be used without restrictions.
Variable displacement pumps generate significant noise. It is known these elements show bad efficiency when only part of the power is required. When idle, the variable displacement pump driven by a constant speed motor still rotates full speed resulting in high idle losses.
The goal of the invention was to find a drive system combining the advantages of hydraulic and electric drive technology. The system must combine high force concentration, high efficiency and easy power distribution. It was also a goal of the invention to make the new drive system applicable to a wide range of dynamic machines such as flight simulators, robots, injection molding machines and presses or die casting machines. All of them need accurate and dynamic control of pressure and/or velocity and/or position.
A hybrid electric and hydraulic system, comprising a variable speed electric motor, a pump, a hydraulic actuator, and an open hydraulic circuit that includes a tank, the motor being arranged to drive the pump to suction from the tank, the pump being in fluid communication with the hydraulic actuator so as to vary pressure and/or speed and/or position of the hydraulic actuator in accordance with a speed of the electric motor.
A hybrid electric and hydraulic system for the actuation of a dynamic machine axis, comprising a variable speed electric motor, a pump, an open hydraulic circuit that includes a tank, a hydraulic cylinder and a hydraulic accumulator, the motor being arranged to drive the pump to suction from the tank, the hydraulic cylinder having a piston movable between retracted and extended positions within the hydraulic cylinder, the pump being in fluid communication with the hydraulic cylinder on one side of a head of the piston, the hydraulic cylinder being in fluid communication with the hydraulic accumulator on another side of the head of the piston so that hydraulic energy routes to the accumulator as the piston moves in one direction and the accumulator supplies energy for effecting a reverse direction of motion of the piston.